Chapter 6 Single-Dimensional Arrays. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved.

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1 Chapter 6 Single-Dimensional Arrays rights reserved. 1 Opening Problem Read one hundred numbers, compute their average, and find out how many numbers are above the average. Solution AnalyzeNumbers rights reserved. 2

2 Objectives To describe why arrays are necessary in programming ( 7.1). To declare array reference variables and create arrays ( ). To obtain array size using arrayrefvar.length and know default values in an array ( 7.2.3). To access array elements using indexes ( 7.2.4). To declare, create, and initialize an array using an array initializer ( 7.2.5). To program common array operations (displaying arrays, summing all elements, finding the minimum and maximum elements, random shuffling, and shifting elements) ( 7.2.6). To simplify programming using the foreach loops ( 7.2.7). To apply arrays in application development (AnalyzeNumbers, DeckOfCards) ( ). To copy contents from one array to another ( 7.5). To develop and invoke methods with array arguments and return values ( ). To define a method with a variable-length argument ( 7.9). To search elements using the linear ( ) or binary ( ) search algorithm. To sort an array using the selection sort approach ( 7.11). To use the methods in the java.util.arrays class ( 7.12). To pass arguments to the main method from the command line ( 7.13). rights reserved. 3 Introducing Arrays Array is a data structure that represents a collection of the same types of data. rights reserved. 4

3 Declaring Array Variables datatype[] arrayrefvar; Example: double[] mylist; datatype arrayrefvar[]; // This style is allowed, but not preferred Example: double mylist[]; rights reserved. 5 Creating Arrays arrayrefvar = new datatype[arraysize]; Example: mylist = new double[10]; mylist[0] references the first element in the array. mylist[9] references the last element in the array. rights reserved. 6

4 Declaring and Creating in One Step datatype[] arrayrefvar = new datatype[arraysize]; double[] mylist = new double[10]; datatype arrayrefvar[] = new datatype[arraysize]; double mylist[] = new double[10]; rights reserved. 7 The Length of an Array Once an array is created, its size is fixed. It cannot be changed. You can find its size using arrayrefvar.length For example, mylist.length returns 10 rights reserved. 8

5 Default Values When an array is created, its elements are assigned the default value of 0 for the numeric primitive data types, '\u0000' for char types, and false for boolean types. rights reserved. 9 Indexed Variables The array elements are accessed through the index. The array indices are 0-based, i.e., it starts from 0 to arrayrefvar.length-1. In the example in Figure 6.1, mylist holds ten double values and the indices are from 0 to 9. Each element in the array is represented using the following syntax, known as an indexed variable: arrayrefvar[index]; rights reserved. 10

6 Using Indexed Variables After an array is created, an indexed variable can be used in the same way as a regular variable. For example, the following code adds the value in mylist[0] and mylist[1] to mylist[2]. mylist[2] = mylist[0] + mylist[1]; rights reserved. 11 Array Initializers Declaring, creating, initializing in one step: double[] mylist = {1.9, 2.9, 3.4, 3.5; This shorthand syntax must be in one statement. rights reserved. 12

7 Declaring, creating, initializing Using the Shorthand Notation double[] mylist = {1.9, 2.9, 3.4, 3.5; This shorthand notation is equivalent to the following statements: double[] mylist = new double[4]; mylist[0] = 1.9; mylist[1] = 2.9; mylist[2] = 3.4; mylist[3] = 3.5; rights reserved. 13 CAUTION Using the shorthand notation, you have to declare, create, and initialize the array all in one statement. Splitting it would cause a syntax error. For example, the following is wrong: double[] mylist; mylist = {1.9, 2.9, 3.4, 3.5; rights reserved. 14

8 Trace Program with Arrays Declare array variable values, create an array, and assign its reference to values public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the array is created rights reserved. 15 Trace Program with Arrays i becomes 1 public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the array is created rights reserved. 16

9 Trace Program with Arrays i (=1) is less than 5 public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the array is created rights reserved. 17 Trace Program with Arrays After this line is executed, value[1] is 1 public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the first iteration rights reserved. 18

10 Trace Program with Arrays After i++, i becomes 2 public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the first iteration rights reserved. 19 Trace Program with Arrays public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; i (= 2) is less than 5 After the first iteration rights reserved. 20

11 Trace Program with Arrays After this line is executed, values[2] is 3 (2 + 1) public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the second iteration rights reserved. 21 Trace Program with Arrays After this, i becomes 3. public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the second iteration rights reserved. 22

12 Trace Program with Arrays i (=3) is still less than 5. public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the second iteration rights reserved. 23 Trace Program with Arrays After this line, values[3] becomes 6 (3 + 3) public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the third iteration rights reserved. 24

13 Trace Program with Arrays After this, i becomes 4 public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the third iteration rights reserved. 25 Trace Program with Arrays i (=4) is still less than 5 public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the third iteration rights reserved. 26

14 Trace Program with Arrays After this, values[4] becomes 10 (4 + 6) public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the fourth iteration rights reserved. 27 Trace Program with Arrays After i++, i becomes 5 public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the fourth iteration rights reserved. 28

15 Trace Program with Arrays i ( =5) < 5 is false. Exit the loop public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; After the fourth iteration rights reserved. 29 Trace Program with Arrays After this line, values[0] is 11 (1 + 10) public class Test { public static void main(string[] args) { int[] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; values[0] = values[1] + values[4]; rights reserved. 30

16 Processing Arrays See the examples in the text. 1. (Initializing arrays with input values) 2. (Initializing arrays with random values) 3. (Printing arrays) 4. (Summing all elements) 5. (Finding the largest element) 6. (Finding the smallest index of the largest element) 7. (Random shuffling) 8. (Shifting elements) rights reserved. 31 Initializing arrays with input values java.util.scanner input = new java.util.scanner(system.in); System.out.print("Enter " + mylist.length + " values: "); for (int i = 0; i < mylist.length; i++) mylist[i] = input.nextdouble(); rights reserved. 32

17 Initializing arrays with random values for (int i = 0; i < mylist.length; i++) { mylist[i] = Math.random() * 100; rights reserved. 33 Printing arrays for (int i = 0; i < mylist.length; i++) { System.out.print(myList[i] + " "); rights reserved. 34

18 Summing all elements double total = 0; for (int i = 0; i < mylist.length; i++) { total += mylist[i]; rights reserved. 35 Finding the largest element double max = mylist[0]; for (int i = 1; i < mylist.length; i++) { if (mylist[i] > max) max = mylist[i]; rights reserved. 36

19 Random shuffling you need to randomly reorder the elements in an array. This is called a shuffling. To accomplish this, for each element mylist[i], randomly generate an index j and swap mylist[i] with mylist[j], as follows: for (int i = 0; i < mylist.length - 1; i++) { // Generate an index j randomly int j = (int)(math.random() * mylist.length); // Swap mylist[i] with mylist[j] double temp = mylist[i]; mylist[i] = mylist[j]; mylist[j] = temp; A random index i mylist [0] [1] [i] [j].... swap rights reserved. 37 Shifting Elements Sometimes you need to shift the elements left or right. Here is an example to shift the elements one position to the left and fill the last element with the first element: rights reserved. 38

20 Enhanced for Loop (for-each loop) JDK 1.5 introduced a new for loop that enables you to traverse the complete array sequentially without using an index variable. For example, the following code displays all elements in the array mylist: for (double value: mylist) System.out.println(value); In general, the syntax is for (elementtype value: arrayrefvar) { // Process the value You still have to use an index variable if you wish to traverse the array in a different order or change the elements in the array. rights reserved. 39 Analyze Numbers Read one hundred numbers, compute their average, and find out how many numbers are above the average. AnalyzeNumbers Run rights reserved. 40

21 Copying Arrays Often, in a program, you need to duplicate an array or a part of an array. In such cases you could attempt to use the assignment statement (=), as follows: 2 = 1; rights reserved. 41 Copying Arrays This statement (2 = 1;) does not copy the contents of the array referenced by 1 to 2, But, copies the reference value from 1 to 2. After this statement, 1 and 2 reference to the same array. The array previously referenced by 2 is no longer referenced; it becomes garbage, which will be automatically collected by the Java Virtual Machine. rights reserved. 42

22 Copying Arrays In Java, you can use assignment statements to copy primitive data type variables, but not arrays. There are three ways to copy arrays: Use a loop to copy individual elements one by one. Use the static arraycopy method in the System class. Use the clone method to copy arrays; this will be introduced in Chapter 14, Abstract Classes and Interfaces. rights reserved. 43 Copying Arrays (Using a loop) int[] sourcearray = {2, 3, 1, 5, 10; int[] targetarray = new int[sourcearray.length]; for (int i = 0; i < sourcearrays.length; i++) targetarray[i] = sourcearray[i]; rights reserved. 44

23 The arraycopy Utility arraycopy(sourcearray, src_pos, targetarray, tar_pos, length); Example: System.arraycopy(sourceArray, 0, targetarray, 0, sourcearray.length); rights reserved. 45 Passing Arrays to Methods public static void printarray(int[] array) { for (int i = 0; i < array.length; i++) { System.out.print(array[i] + " "); Invoke the method int[] = {3, 1, 2, 6, 4, 2; printarray(); Invoke the method printarray(new int[]{3, 1, 2, 6, 4, 2); Anonymous array rights reserved. 46

24 Pass By Value rights reserved. 47 Simple Example rights reserved. 48

25 Call Stack When invoking m(x, y), the values of x and y are passed to number and numbers. Since y contains the reference value to the array, numbers now contains the same reference value to the same array. rights reserved. 49 Call Stack When invoking m(x, y), the values of x and y are passed to number and numbers. Since y contains the reference value to the array, numbers now contains the same reference value to the same array. rights reserved. 50

26 Heap Heap Space required for the main method int[] y: reference int x: The arrays are stored in a heap. The JVM stores the array in an area of memory, called heap, which is used for dynamic memory allocation where blocks of memory are allocated and freed in an arbitrary order. rights reserved. 51 Passing Arrays as Arguments Objective: Demonstrate differences of passing primitive data type variables and array variables. TestPassArray Run rights reserved. 52

27 Example, cont. Stack Space required for the swap method n2: 2 n1: 1 Space required for the main method int[] a reference Invoke swap(int n1, int n2). The primitive type values in a[0] and a[1] are passed to the swap method. Heap a[1]: 2 a[0]: 1 The arrays are stored in a heap. Stack Space required for the swapfirsttwoinarray method int[] array reference Space required for the main method int[] a reference Invoke swapfirsttwoinarray(int[] array). The reference value in a is passed to the swapfirsttwoinarray method. rights reserved. 53 Returning an Array from a Method public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); rights reserved. 54

28 Trace the reverse Method int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; Declare and create array rights reserved. 55 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i = 0 and j = rights reserved. 56

29 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i (= 0) is less than rights reserved. 57 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i = 0 and j = 5 Assign [0] to [5] rights reserved. 58

30 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; After this, i becomes 1 and j becomes 4 i <.length; i++, j--) { [j] = [i]; rights reserved. 59 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i (=1) is less than 6 i <.length; i++, j--) { [j] = [i]; rights reserved. 60

31 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i = 1 and j = 4 Assign [1] to [4] rights reserved. 61 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; After this, i becomes 2 and j becomes rights reserved. 62

32 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i (=2) is still less than rights reserved. 63 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i = 2 and j = 3 Assign [i] to [j] rights reserved. 64

33 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; After this, i becomes 3 and j becomes rights reserved. 65 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i (=3) is still less than rights reserved. 66

34 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i = 3 and j = 2 Assign [i] to [j] rights reserved. 67 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; After this, i becomes 4 and j becomes rights reserved. 68

35 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i (=4) is still less than rights reserved. 69 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i = 4 and j = 1 Assign [i] to [j] rights reserved. 70

36 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; After this, i becomes 5 and j becomes rights reserved. 71 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i (=5) is still less than rights reserved. 72

37 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i = 5 and j = 0 Assign [i] to [j] rights reserved. 73 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; After this, i becomes 6 and j becomes rights reserved. 74

38 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; i (=6) < 6 is false. So exit the loop rights reserved. 75 Trace the reverse Method, cont. int[] 1 = {1, 2, 3, 4, 5, 6; int[] 2 = reverse(1); public static int[] reverse(int[] ) { int[] = new int[.length]; i <.length; i++, j--) { [j] = [i]; Return rights reserved. 76

39 Problem: Counting Occurrence of Each Letter Generate 100 lowercase letters randomly and assign to an array of characters. Count the occurrence of each letter in the array. (a) Executing createarray in Line 6 Stack Space required for the createarray method char[] chars: ref Space required for the main method char[] chars: ref Heap Array of 100 characters (b) After exiting createarray in Line 6 Stack Space required for the main method char[] chars: ref Heap Array of 100 characters CountLettersInArray Run rights reserved. 77 Problem: Calculator Objective: Write a program that will perform binary operations on integers. The program receives three parameters: an operator and two integers. java Calculator java Calculator 2-3 Calculator Run java Calculator 2 / 3 java Calculator 2. 3 rights reserved. 78